• An expression that can be replaced with its value and vice versa without changing the behavior of the program.
• Put another way, a function is referentially transparent if for a given input it returns the same output every time, i.e. a lookup table.

int x = 0;
int foo(int y) {
    return y * 2;
}
int bar(int y) {
    return x++ * y;
}
int main() {
    printf("x = %d\n", x);
    int z = 2;
    int e1 = foo(z) + z - (foo(z) - foo(z));
    printf("x = %d, e1 = %d\n", x, e1);
    int e2 = foo(z) + z - (bar(z) - bar(z));
    printf("x = %d, e2 = %d\n", x, e2);
    return 0;
}